Dihalo- and trihalopyridines and especially 2,5-dihalo- and 2,5,6-trihalopyridines are well known bacterials, fungicides and pesticides and in addition these compounds are useful as intermediates in the preparation of other pesticides and antiviral and antibacterial agents. Such uses are taught in many patents including U.S. Pat. Nos. 3,244,586, 1,778,784 and 3,420,833.
These compounds can be prepared by a number of methods including halogen exchange, and high pressure chlorination and bromination processes. Many of the presently employed methods for the preparation of these compounds use pyridine as a starting material and are expensive, or time consuming, or give undesirably low yields. It has become apparent that a method for preparing the 2,5-dihalo and 2,5,6-trihalopyridines from the more readily available tetrahalo- and pentahalopyridines would be advantageous.
Collins et al, J. Chem. Soc., (c); pages 167-174 (1971) teach that halogens ortho and para to the ring nitrogen in pentahalopyridines are reactive with hydrazine hydrate. This reference further teaches the formation of tetrahalo-4-hydroxypyridines from the action of aqueous sodium hydroxide on tetrahalo-4-hydrazino pyridines.